C4D (Complement Component 4) in Plasma or Serum
This test measures the amount of C4 protein, a part of your immune system’s complement system, which helps fight infections and controls inflammation.
["C4 helps identify immune system function and potential autoimmune conditions."]
Key facts at a glance
- LOINC
- 21217-5
- Short code
- C4d
- Synonyms
- C4d Plas-mCnc
- Category
- chemistry
- Sample
- plasma
- Container
- EDTA plasma tube (lavender top) or citrate plasma tube (light blue top)
- Units
- N/A
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- 18–65 years
- Method
- Enzyme-Linked Immunosorbent Assay (ELISA)
- CPT
- 86161
- Specimen volume
- 1-2 mL of plasma
- Stability
- Plasma stable for 8 hours at room temperature; 2 days at 2-8°C; up to 3 months at -20°C or colder for long-term storage
- Hemolysis max
- 100
- Lipemia max
- 100
- Icterus max
- 50
- TAT
- 24–72 hours
- Order
- routine
What Is C4 Used For?
Doctors may order a C4 test to assess how well your immune system is functioning, especially in cases of suspected autoimmune diseases like lupus or other inflammatory conditions. It can also help monitor treatment progress for these conditions.
How Is the Test Performed?
A blood sample is drawn from a vein, usually in your arm. The blood is then processed to separate plasma or serum for testing. The test measures the concentration of C4 protein in the sample.
What Does a C4 Test Tell You?
Lower than expected levels of C4 may suggest an autoimmune disorder, chronic infection, or other conditions affecting the complement system. Higher levels might indicate certain inflammatory responses or lab artifacts, so results should be interpreted by your healthcare provider.
When Might You Need This Test?
You might need a C4 test if you have symptoms of autoimmune diseases (e.g., joint pain, fatigue), if you’re being treated for an infection or inflammation, or if your doctor suspects a condition affecting your immune system.
Reference range
Reference ranges vary by laboratory and assay methodology; consult your local lab's reference interval guide.N/A
No fasting required. Patient should be well-hydrated prior to specimen collection.
Clinical interpretation
How abnormal results are interpreted and what conditions they suggest.
Clinical Significance: C4 plays a critical role in the immune response by enhancing antibody activity and promoting inflammation. Abnormal levels can signal underlying immune dysfunction, chronic inflammation, or autoimmune activity, guiding treatment decisions.
Short name
C4 Level
Frequently asked questions
Why might my C4 test result be different from what I expected?
Can diet or lifestyle affect my C4 test results?
Is a C4 test painful?
Sources
- 1
- 2Lab Tests OnlineSource
- 3
Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.
This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.
This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.
If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.
Medically reviewed by WellAlly Editorial Team Last reviewed August 15, 2026. · Read our editorial policy